The police usually come late, if at all.
=The police usually come later. Usually, they don't even come.
Use alcohol in moderation, if at all.
=Use alcohol in moderation, or don't use alcohol.
when 이나 how와 같이 쓰이면, 두 개의 질문을 한번에 하는 것임.
When, if at all, will the war end?
= When will the war end, if it does end at any time?